Tuesday, May 5, 2009

Climate Change: Caused by the Sun?

Global warming skeptics claim that changes in earth's temperature can be explained by the natural variability of the sun and that man-made greenhouse gases like carbon dioxide are not a significant factor.

I've been trying to investigate this claim. Here's what I found out so far:

Scientists who believe that recent global warming is caused by man-made CO2 concede that that the average global temperature is correlated with the sun's output until about 1975. But they argue that the correlation breaks down after that. That is certainly the case in this graph from the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder, which compares global temperatures with the amount of energy flowing from the sun.

Source: http://lasp.colorado.edu/science/solar_influence/index.htm

As you can see, starting in the '70s the earth's temperature increases faster than the sun's output. The argument goes that some source other than the sun, namely CO2, must be responsible for the warming during these years.

This is a strawman argument, though, since the skeptics do not rely on solar irradiance to make their case. After all, even before 1975 the correlation is not that good. For example, temperature increased faster than solar irradiance in the early 1900s, even though cars and factories were pumping far less CO2 into the atmosphere than they do today.

Two prominent skeptics, Eigil Friis-Christensen and Henrik Svensmark (Advances in Space Research 20:4-5, pp. 913-921 (1997) http://www-ssc.igpp.ucla.edu/IASTP/43/) propose that sunspot activity, rather than solar irradiance, is what affects earth's temperature. The number of sunspots that appears on the sun's surface rises and falls in cycles of approximately eleven years. Some cycles are a little longer, some a little shorter. As shown in the second graph, the length of the sunspot cycle correlates with global temperature much better than solar irradiance.

However this data only goes until 1980, so it is unclear whether the correlation continues into the last three decades; I have not been able to find anything more recent. Note also that Friis-Christensen and Svensmark do not rule out the possibility that climate change is due to a mixture of solar activity and greenhouse gases.

So there's no smoking gun (or smoking chimney) yet. If anybody can point me to some more recent data on solar activity and climate, or some hybrid models that incorporate both sunspots and CO2, please comment.


Anonymous Juggling Frogs said...

I'm so glad you're looking into this, and eagerly await the results of your further research. Thank you!

May 5, 2009 9:39 PM  
Blogger PeterEverett said...

For the past few years, the scholarship on the solar influence on climate, particularly the variations in the ability of the sun to shield the inner solar system from cosmic rays, has presented a persuasive case that the man-made contribution to climate change is very small when compared to the solar contribution. This fact takes on new relevance and urgency today, as the sun is in a record-breaking low of a character that in the past has precipitated events like the Dalton minimum, with its "year without summer", or even the Maunder minimum, with its "Little Ice Age". One very good blog on the subject belongs to an Israeli physicist, Nir J. Shaviv, at http://www.sciencebits.com/scienceblog . Global cooling is probably the biggest threat today.

The central controversy is how sensitive the climate is to small changes in heat flux. Working Group 1 of the IPCC's Fourth Assessment Report ( http://www.ipcc.ch/ipccreports/ar4-wg1.htm ) adopts a model with high sensitivity, based on the fact that the most significant greenhouse gas is not CO2, but rather H2O, and that warmer air contains more moisture, providing a source of positive feedback. CO2 by itself, at relevant concentrations, has a greenhouse effect that is far to small to account for what the IPCC attributes to it. It requires a "multiplier" and water vapor is offered as the principal multiplier.

One criticism of the IPCC's models it that it is so sensitive to fluctuations in temperature that past CO2 fluctuations would have "run away" in a positive feedback loop many thousands of years ago, which is obviously not seen in the record. Another criticism is one that the IPCC itself calls the largest uncertainty in their model, which is the potential negative feedback effects of low clouds.

Enter Henrik Svensmark. Since 1997, Svensmark has been arguing that most of the historical climate record, both ancient and recent, is explainable in terms of the modulating effects of the solar cycle on cosmic ray flux in the atmosphere. (See H. Svensmark. Cosmoclimatology: a new theory emerges. Astronomy & Geophysics 48 (1): 1.18–1.24 http://www.blackwell-synergy.com/doi/full/10.1111/j.1468-4004.2007.48118.x ) A number of scientists, including Willie Soon of the Harvard-Smithsonian center for astrophysics, have produced research that confirms the solar cycle, particularly as it is recorded in proxies for cosmic ray flux, like Beryllium-10 and Carbon-14, is statistically a far better explanation for global temperature changes than is carbon dioxide.

Like carbon dioxide, the variation in the total solar irradiance by itself is far too small to explain the observed temperature changes. It too needs a "multiplier", which according to Svensmark comes in the form of the portion of the sun's output that deflects cosmic rays away from Earth. This varies by much more than the 0.1% of total solar irradiance. The part of the atmosphere that would be most sensitive to cloud formation due to increased cosmic ray ionization of the air is exactly the part to which the IPCC uses to amplify the small effects of man-made carbon dioxide, namely the moisture-laden air over warm tropical oceans. But for lack of nucleii for droplet formation, clouds would readily form. Ions produced by cosmic rays are said to form these nuclei.

The view of the man-made global warming skeptics, which appears sensible, is that the climate is less sensitive, and more stable than the IPCC holds it to be, and that this stability is manifest in the historical record, which includes periods that are both much warmer and much colder, and with much more CO2 than today. However, this relative stability can still be influenced by indirect solar modulation of cloud formation.

The cruel joke that Nature appears to have played on climate scientists is that the Sun has been in a period of very high activity that rose from the Dalton minimum in the 1800s to a great crescendo in 1957, and has only recently started to subside. That this rising solar activity almost exactly parallels the Industrial Revolution and the increasing burning of fossil fuels has probably led many people astray in ascribing cause and effect.

Now that we are in a period of very unusual solar quiet, the solar wind is at its lowest recorded level, and cosmic ray flux is at its highest. This not only has significance for climate, but also space exploration, as manned missions outside the Earth's protective magnetic field will be exposed to radiation from galactic cosmic rays at an all-time high, as will electronic components which can not be effectively shielded from these super-high energy particles.

The coming decade will probably prove to even the biggest skeptics who is right about climate, especially if the Sun persists in its current quiet ways. I hope our politicians don't cripple the global economy further in the name of reducing the output carbon dioxide, which is not a pollutant, and only weakly influential on climate. Although it is less than 0.05% of the atmosphere, carbon dioxide is a most precious resource. Every bit of carbon in our bodies was once CO2 in the atmosphere which nourished a plant, and then eventually us.

May 5, 2009 11:22 PM  

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